1. Field of the Invention
The present invention relates to a composite powder of silicon nitride and silicon carbide, a method of manufacturing such a composite powder, and a method of manufacturing a sintered composite body of silicon nitride and silicon carbide which has a nanocomposite structure.
2. Description of the Prior Art
Sintered ceramic bodies of silicon nitride are expected to find applications for use as structural ceramics under severe conditions at high temperatures because of their high strength, high heat resistance, high thermal shock resistance, high wear resistance, and high acid resistance. While various compounds are added to their materials with a view to improving the mechanical strength of the sintered ceramic bodies, silicon carbide is one of the most promising ceramic compounds for making the sintered ceramic bodies highly resistant to acids, strong at high temperatures, and mechanically strong. Therefore, there have been proposed sintered ceramic bodies with silicon carbide added to silicon nitride. However, those sintered ceramic bodies which are composed of a simple mixture of powders of silicon nitride and silicon carbide do not have a nanocomposite structure, but is of such a structure in which the powder of silicon carbide with its particles having sizes on the order of microns exist only in the grain boundary of silicon nitride. There have been proposed various processes for producing sintered ceramic bodies having a nanocomposite structure in which particles of silicon carbide are finely dispersed in particles of silicon nitride.
Japanese laid-open patent publication No. 2-160669 discloses a sintered composite body of silicon nitride and silicon carbide which has a microstructure wherein particles of silicon carbide having an average diameter of 1 .mu.m or less are dispersed in the grain boundary and fine particles of silicon carbide having sizes ranging from several nanometers to several hundred nanometers are dispersed in particles of silicon nitride. The disclosed sintered composite body is produced by firing, at a temperature ranging from 1500.degree. to 2300.degree. C. in the presence of a sintering additive, a composite powder of amorphous silicon nitride and silicon carbide or a mixed powder of silicon nitride and silicon carbide which produces fine particles of silicon nitride having an average diameter of 0.5 .mu.m or less in a liquid phase sintering system. However, the composite powder of amorphous silicon nitride and silicon carbide, which is produced by a vapor phase reaction of an organic silicon compound, is highly costly to manufacture and cannot efficiently be mass-produced. Another problem is that the amorphous powder has poor moldability. The mixed powder of silicon nitride and silicon carbide which is specifically disclosed is a mixture of a composite powder of amorphous silicon nitride and silicon carbide and a powder of silicon carbide, and hence suffers the same problems as the composite powder of amorphous silicon nitride and silicon carbide.
Japanese laid-open patent publication No. 3-261611 discloses a process of manufacturing a composite powder of silicon nitride to be used in the manufacture of a sintered composite body of silicon nitride and silicon carbide. According to the disclosed process, a metal powder of silicon (Si) and a carbonaceous powder are mixed with each other, and the mixture is heated in an inert gas atmosphere containing nitrogen at a temperature of 1400.degree. C. or less for thereby simultaneously effecting carbonizing and nitriding reactions of the metal powder of silicon. However, since the composite powder contains no sintering additive, .beta.-type silicon nitride tends to separate out of the composite powder, making it difficult to increase the content of .alpha.-type silicon nitride in the composite powder. The disclosed process is also problematic in that the produced powder has large particle diameters and hence poor sinterability because the composite powder undergoes grain growth during the reactions.